This is what I try to do when guesstimating as well. I don't know any other way, really, than "player X with comparable skills/talent went recently for €Z to club Y". I don't have a degree in finance, so this is the only way I can think of being at least close to accurate.
But then you have this guy (and many others as well), just disregarding recent transfers totally and comes up with his own numbers:
I don't mean to put you on the spot or anything
@crossy1686 , you're just the most recent example I could find of someone completely disregarding recent transfer history and coming up with numbers totally on your own.